A radio communication apparatus needs to increase transmission power to transmit signals so as to enable a communicating party to receive the signals, but when the power is excessively increased, the radio communication apparatus interferes with radio communication apparatuses other than the communicating party. Therefore, by performing transmission power control, the power is adjusted to suitable transmission power whenever necessary.
In general, as described in Patent Document 1, a transmission power control apparatus uses a variable amplifying circuit with an analog control voltage as its input voltage. The variable amplifying circuit has the gain characteristic proportional to the analog control voltage generated by a D/A converter. However, when a wide dynamic range is required as in a CDMA (Code Division Multiple Access) scheme, it is difficult to maintain linearity of the input/output characteristic (control voltage-gain characteristic) over the entire voltage control range, which is described below with reference to drawings.
FIG. 1 is a graph showing the gain characteristic of a variable amplifying circuit with respect to the analog control voltage. As shown in the figure, in gain control with the analog voltage, the gain shows nonlinearity when the control voltage (DAC code) is low or high, and the control voltage-gain characteristic is not uniform over a wide dynamic range. Therefore, in a conventional transmission power control apparatus, the DAC code for arbitrary transmission power is stored in memory, and read out when necessary to control an amplification rate, and it is thereby possible to obtain accurate transmission power. In addition, the resolution of transmission power is determined by the resolution of the D/A converter and control sensitivity characteristics of the variable amplifying circuit.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI11-177444